The mechanism of the magnetic sensitivity of fiber optic gyroscope (FOG) is introduced and experimental study is done. The Helmholtz coils equipment and FOG test platform is utilized, the magnetic sensitivity of FOG in direct current (DC) magnetic field is studied, and the linear relation of FOG data output and the magnetic flux density is found. Axial magnetic field and radial magnetic field influence coefficients and the orientation of the magnetic sensitivity axis of FOG are also found. Then the influence on FOG bias and bias stability and the alternating current (AC) magnetic field sensitivity in 50 Hz AC magnetic field are mainly analyzed, finding that the influence of AC magnetic field on the bias can be neglected, but the magnetic flux density and the size of FOG bias stability are almost linear relation. Finally the influence of different frequency AC magnetic fields on the FOG data output is studied, indicating that the data output of FOG is mainly proportional to the transient magnetic flux density, different frequency AC magnetic fields have different influences on the bias of FOG. There is a frequency (or frequency range) with much stronger influence on the bias of FOG, which is related to FOG output bandwidth.